You understand that Tableau is NOT a desktop publishing/graphics design program, right?
Do it in PowerPoint.
Yes...thank you Shawn for your useless commentary.
You're welcome :)
I suggest copying as an image and then using Powerpoint to put the border on (in Picture tools / Format). Alternatively if you're going to print and want a high-quality image you might find that printing to pdf then using acrobat to capture an image at high dpi might get you something better.
I've tried in the past printing a pdf and importing it into illustrator / other vector drawing programs for fine-tuning but it seems like lots of work - the images aren't particularly 'clean'.
There is a way of getting server to deliver a pdf through the URL - maybe somebody else can explain that / you can find it in an old conversation.